Twenty-three years after the Exxon Valdez spilled more than half a million barrels of oil into Prince William Sound, another threat looms over Alaska’s remote and beautiful coastline — in the form of heavy oil exports from Canada to China.

Since the Earth is a sphere, the shortest shipping route from Western Canada to China passes through the Aleutian Islands at a narrow strait called Unimak Pass.

Two pipeline companies want to dilute tar-like bitumen from the Alberta oil sands with natural gas condensate so that it can be pumped west to the coast of British Columbia.

The first plan — a new pipeline called “Northern Gateway” — would carry 525,000 barrels per day to a terminal just south of the Alaska Panhandle, where it would be loaded onto supertankers that would sail westward toward Unimak Pass.

The second plan involves tripling the capacity of an existing pipeline to Vancouver so it can carry 850,000 barrels per day, and adding compressor stations so it can handle the diluted but still heavy bitumen. The oil from this “Trans Mountain Pipeline” would also be shipped through Unimak Pass.

Unimak Pass is just 10 miles wide. Five thousand ships already use it each year, most of them large container and bulk-cargo vessels.

The tidal mixing of cold nutrient-rich waters in and around Unimak Pass supports massive amounts of plankton, the basis of a rich food chain. The area is part of the Alaska Maritime National Wildlife Refuge, which is home to 40 million seabirds. It’s also home to a wealth of marine mammals, including endangered Steller sea lions, northern fur seals, sea otters and numerous species of whales.

This ecosystem has considerable economical value. The Bering Sea just north of Unimak Pass supports the largest commercial fishery in the United States, worth $1 billion annually.

Severe weather and sea conditions are common in Unimak Pass, along with powerful tidal flows. In December 2004, the Selendang Ayu, a 738-foot-long Malaysian cargo ship, had just cleared the pass when it lost power in a storm. The vessel was blown aground and broke apart, spilling 335,000 gallons of fuel oil. Almost none of the oil was recovered due to the remote location, bad weather and the near-complete absence of oil-spill-cleanup equipment and personnel in the Aleutians.

Complicating matters, the U.S. State Department has long accepted that Unimak Pass is an “international strait” that foreign vessels can enter without permission or regulatory restriction. As a result, there are no shipping lanes, or notification or pilotage requirements.

There are a few steps the federal government could take. It could station a large rescue tug and several oil-spill-cleanup vessels at nearby Dutch Harbor. It could ask the International Maritime Organization to designate Unimak Pass as a “particularly sensitive sea area,” which would enable the U.S. to require advance notification of passage and adherence to vessel traffic separation rules. It could seek to persuade shipping companies to voluntarily route oil tankers well south of the Aleutians, though this would increase both distance and cost.

In the end, however, none of these steps is likely to prevent hundreds of oil tankers from transiting Unimak Pass each year. For the root of the problem is not the tankers, but Canada’s disregard for the environmental impacts of developing and selling its oil sands to China — impacts that include the near-inevitability of another Exxon Valdez-type spill in U.S. waters, this time in Unimak Pass.

Michael Byers holds the Canada Research Chair in Global Politics and International Law at the University of British Columbia in Vancouver. Prior to 2005 he was a professor of law at Duke University.